DNA topoisomerases are the enzymes responsible for maintaining and controlling the topological state of DNA in the cell. They play an important role in replication, transcription and genetic recombination. Despite their importance, an understanding of their structure and detailed mechanism of action is just emerging. This project is concerned with the study of the structure of Escherichia coli DNA topoisomerases I and III. We have used SSRL data to study the structures of the 67 and the 30 kDa fragments of E. coli DNA topoisomerase I including complexes of the 67 kDA fragment with short oligonucleotides. We also collected data on the intact E. coli DNA topoisomerase III. We solved the structure of topoisomerase III by MIR using data collected at SSRL and we are currently refining the structure. The latter project could not be done without access to synchrotron radiation due to the size of the crystals.